Males with sickle cell disease have higher risks of cerebrovascular disease, increased inflammation, and a reduced response to hydroxyurea
Authors: Margherita Di Mauro, Sara El Hoss, Amina Nardo‐Marino, Sara Stuart‐Smith, John Strouboulis, John S. Gibson, David C. Rees, John N. Brewin
Journal: American Journal of Hematology (2023)

Biological sex is important. Male sex is associated with worse outcomes in most measures, including cerebrovascular disease, hospital admissions, and blood transfusions, but not survival. Females also appear to have a better response to hydroxyurea therapy, reduced markers of inflammation, and better liver function. Sickle cell disease (SCD) is one of the most common serious monogenic diseases worldwide.1 It is caused by an amino acid substitution in the β-globin gene that leads to the production of an abnormal hemoglobin called HbS. Despite the uniform genetic origin, the clinical phenotype varies widely, and this variability is determined by multiple factors, including sex. Differences within many diseases arising due to biological sex are well described, and understanding these impacts on symptomology, treatment responses, and overall prognosis is one of the first steps toward personalized medicine.2 Likewise, in SCD, there has been growing attention to sex-based differences. Previous studies have reported increased survival for females compared with males3 as well as differences in HbF levels, cardiovascular dysfunction, psychological issues, genitourinary complications, and hemolysis.4, 5 In this study, we examined a large cohort of adults and children with HbSS and HbSC, to review the effects of sex on clinical outcomes and laboratory measurements. The aim was to extend the understanding of sex differences in outcomes in SCD, and to shed some light on the pathophysiological mechanisms that might underlie this. All data came from the southeast London sickle cell gene bank cohort (London, UK). Written informed consent was obtained through approved study protocols (LREC 01–083, 07/H0606/165, 12/LO/1610, 18/LO/1566, and 11/LO/0065) and research was conducted in accordance with the Helsinki Declaration (1975, as revised 2008). Full methods of data collection for this cohort are described previously6 and in the Appendix S1. We evaluated a total of 1069 individuals: 802 with HbSS and 267 with HbSC. The average age of the individuals with HbSS was 35.1 years (range 2–81 years) while in those with HbSC, it was 45 years (range 13–86 years). In the HbSS cohort, 54% were female, and in the HbSC cohort, 64% were female. The prevalence of α-thalassemia was not different between the sexes. Clinical laboratory differences: Steady-state laboratory measurements, free from hydroxyurea or transfusion, were available for 420 patients with HbSS and 148 patients with HbSC. The full results are shown in Table S1. Male sex was associated with a significant increase in total hemoglobin, hematocrit, red blood cell (RBC) count and absolute reticulocyte count (ARC) in both individuals with HbSS (p = .002, p = .012, p = 2 × 10−5, and p = 5.6 × 10−6 respectively) and HbSC (p = 2.5 × 10−16, p = 1.4 × 10−16, p = 2.7 × 10−11, and p = 3 × 10−5 respectively). Erythropoietin (EPO) levels were available for 231 individuals with HbSS and 118 patients with HbSC. We found no influence of sex on EPO values. HbF measurements were significantly higher in females with HbSS compared with males (p = 6.9 × 10−12). Total white cell count (WCC) was lower in females with both HbSS and HbSC, although neither finding was statistically significant after correction for multiple testing. Eosinophils and monocytes were significantly lower in females with HbSS (p = 0.0004 and p = 3.9 × 10−8) and HbSC (p = .0002 and p = 1.6 × 10−7). C-reactive protein (CRP) levels were also significantly lower in females with HbSS, compared with the males (p = 4.4 × 10−6). Males with HbSS had significantly increased levels of bilirubin, aspartate aminotransferase (AST), and lactate dehydrogenase (LDH), although LDH differences were not statistically significant after correction for multiple testing (p = .0002, p = 6 × 10−5, and p = .05 respectively). In addition, alkaline phosphatase (ALP) and gamma-glutamyl transferase (GGT) were also significantly higher in males with HbSS compared with females (p = 4.9 × 10−6 and p = .005). Estimated glomerular filtration rate (eGFR) was significantly higher in males with HbSS (p = 1.7 × 10−16) but not HbSC (p = .02). Plasma potassium levels were significantly higher in males with both HbSS and HbSC (p = 9.8 × 10−6 and p = 1.6 × 10−6) although there were no significant sex differences in levels of plasma sodium and urine albumin: creatinine ratios (uACR). Red cell cation transport systems: Red cell cation measurements were available for 151 individuals (79 with HbSS and 72 with HbSC). Psickle, KCC cotransport and Gardos channel activities did not vary with sex in HbSS. In HbSC, the KCC cotransport activity was significantly higher in males compared with females (p = .03), Table S2. Response to hydroxyurea therapy: 171 individuals with HbSS had steady-state blood results available both on and off hydroxyurea therapy. The increase in %HbF, total hemoglobin, and MCV associated with hydroxyurea use was significantly greater in females compared with males (p = .00098, p = .046, and p = .012 respectively), Table S3. Cerebrovascular outcomes: 410 individuals with HbSS (69 with overt ischemic strokes (OIS) 105 with large vessel disease (LVD)) and 100 with HbSC (11 with OIS; 12 with LVD) had the necessary clinical data available to be included in the analysis of cerebrovascular outcomes. Males had a higher incidence of all adverse cerebrovascular outcomes (Figure 1A–C) although this was not statistically significant for OIS due to small numbers. Cox proportionate hazard ratios, including ß-globin genotype, and α-thalassemia as covariates, showed the hazard ratio for male sex to be 1.4 (95% CI 0.89–2.2, p = .135) for OIS, 1.5 for LVD (95% CI 1.04–2.17, p = .028), and 1.55 for silent cerebral infarcts (SCI) (95% CI 1.2–2.03, p = .002). Transcranial Doppler measurements (TCD) were available for 243 children (118 female and 125 male), with a total of 1752 scans performed. We found no difference in TCD velocities between males and females with HbSS (Figure 1D), and no difference in the number of children recording at least one abnormal category TCD measurement. Hospital admissions, blood transfusions, and overall survival: 477 individuals with HbSS and 139 with HbSC had more than 2 years of follow-up at KCH. The average number of admissions per year was higher for males with HbSS than females (p = .04), but the number of inpatient days per year was the same (p = .58). For individuals with HbSC, we found no effect of sex on either admission per year or inpatient days per year (Table S4). Sufficient transfusion records were available for 443 individuals with HbSS and 104 with HbSC. Males with HbSS had significantly more transfusion episodes per year and average number of RBC units used per year compared with females (p = .029 and p = .0069). However, when excluding those on a regular transfusion program, there was no difference seen. We observed no significant sex differences in transfusion in the HbSC cohort (Table S4). A total of 899 individuals accounted for 14 662 patient-years of observation and a mean follow-up of 16 years. Among 692 individuals with HbSS, there were 68 deaths at an average age of 41.5 years. Among 207 individuals with HbSC, there were 14 deaths at an average age of 52.5 years. The median survival for these two patient groups was 62 and 80 years, respectively, and we found no effect of sex on survival in either cohort (p = .77) (Figure 1E,F respectively). This study considers the various effects of sex in a large cohort of individuals with HbSS and HbSC disease. We found males with HbSS have worse outcomes in nearly every aspect of the condition, apart from overall survival. Compared with females, males have higher reticulocyte counts, increased inflammation, more abnormal LFTs and renal function, lower HbF levels, less response to hydroxyurea, increased hospital admissions, increased blood transfusions, and increased risk of cerebrovascular disease. It is not clear what underlies most of these differences. Some changes may mirror fundamental differences in biology related to sex seen in the general population, and a limitation of our study is the lack of an ethnically matched disease free cohort for comparison. However, some changes are likely specific to SCD and may stem predominantly from the higher HbF levels seen in females, and possibly the increased nitric oxide availability previously demonstrated.7 In addition, some of the clinical outcomes, particularly hospital admissions, which contrast with previous findings,4 may be influenced by the improved response to hydroxyurea seen in females, which may be related to the improved response seen in those with higher baseline HbF,8 or possibly increased adherence. Lower hemoglobin levels are associated with some adverse outcomes in SCD, including cerebrovascular disease, renal impairment, pulmonary vasculopathy, and mortality.9 However, these are also significantly more common in males, even though they are consistently found to have higher hemoglobin.4, 5 This may suggest that male sex presents some additional risk factors for these that overcome any protective effects of higher hemoglobin. We did not find a sex-based difference in overall survival in our cohort. Although contrasting with previous large studies,3 this is similar to more recent reports in high-income countries.10 It is unclear why the increased morbidity males experience does not translate into reduced mortality and requires further study. The effect of sex on HbSC was much less marked, possibly due to smaller numbers in this study and a lower incidence of complications. Overall, sex has a significant effect on many aspects of HbSS and should be considered in the interpretation of clinical outcomes and laboratory results, and in stratifying the risk of complications in clinical trials and observational studies. Understanding how sex affects these outcomes may shed important new light on the pathophysiology of SCD and lead to improved clinical management. Action Medical Research. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request. Appendix S1. Supporting Information. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.